An electron gun with its collector at a potential of $100 \;V$ fires out electrons in a spherical bulb containing hydrogen gas at low pressure $(10^{-2}\: mm \: of\: Hg)$ A magnetic field of $2.83 \times 10^{-4} \: T$ curves the path of the electrons in a circular orbit of radius $12\; cm$. Find e/m of the electron.